This invention relates to a peak detector and, more particularly, to a peak detector having rise-time enhancement of input signals.
Peak detector circuits are used in various applications wherein it is desired, or necessary, to sense or be provided with an indication of the peak levels of an input signal. A typical peak detector circuit is formed of a diode connected to a capacitor, whereby the capacitor can be charged unidirectionally in response to increasing levels in the input signal applied to the diode. Generally, this type of peak detector has a relatively low rise-time constant so that the capacitor can be rapidly charged in response to increasing signal levels; but has a relatively long fall-time constant so that the capacitor is discharged at a relatively slow rate when the instantaneous input signal level falls below the level stored by the capacitor.
In one typical application of such a peak detector, a peak level meter is used having, for example, an indicating needle which is deflected in accordance with the output voltage of the peak detector. One use of such a peak level meter is to provide indications, or measurements, of the level of a sound generating signal. Such a meter can advantageously be used in sound reproducing systems, such as high fidelity, stereophonic sound processing apparatus.
When the peak level meter is used to indicate sound levels, it is necessary for the indicating needle to reliably follow variations in the peak level of the sound signal, even when the sound signal is a music signal whose level is subject to abrupt changes. In view of the mechanical time constants of the indicating needle and its associated energizing devices, it is appreciated that the needle often cannot follow such abrupt changes in the sound signal. That is, the peak levels of the signal might change too rapidly for the meter to correspondingly follow. Accordingly, it would be necessary to provide a driving circuit for the peak level meter so that abrupt changes of the input signal are enhanced over the normal, or slowly varying input signal. It is thought that if the abrupt changes are so enhanced, or additionally amplified, the indicating needle would be driven in response to such higher level signals, notwithstanding its relatively high mechanical time constant.
A proposed driving circuit for such a peak level meter would include an amplifier, such as an operational amplifier, for supplying the voltage stored on the capacitor of the peak detector to the meter. If a capacitor is provided in the output circuit of the amplifier, abrupt changes in the input signal are detected and a voltage component proportional to such abrupt change is added to the amplified input signal. Thus, during such abrupt changes, the meter would be provided with a first component proportional to the abruptly changed input signal and a second component, added to the first, which is proportional to the abrupt change itself.
Unfortunately, although the abrupt change in the input signal is enhanced by such a peak detector, if the enhancement of the rise-time portion is to be changed, for example, to accommodate faster changes, the characteristics of the entire meter driving circuit are likewise varied. Such variation would require compensation, as by adjusting other parameters so that, in the absence of abrupt changes, the meter nevertheless provides an indication which is an accurate representation of the input signal levels. Consequently, it would be necessary to adjust various circuit parameters for each adjustment in the rise-time enhancement circuit.